The agreement was first reported Wednesday by The Boston Globe, and subsequently confirmed to ESPN.com by a team source. Neither terms nor financial details were yet available, but the deal is believed to be for one year.
Lenon, 31, led the Detroit Lions in tackles last season, starting all 16 games at middle linebacker in a 4-3 alignment. With the Patriots, he will play inside linebacker in New England's 3-4 scheme.
Although starters Jerod Mayo and Tedy Bruschi return at inside linebacker, New England was rendered thin at the position when 2009 third-round draft pick Tyrone McKenzie was lost for his entire rookie season, after suffering a knee injury during minicamp. The Pats also waived 2008 sixth-round choice Bo Ruud, another inside linebacker, last week.
New England officials recently visited with Lenon and free agent Pia Tinoisamoa, who was released by the St. Louis Rams two weeks ago despite leading the team in tackles last season. The addition of Lenon probably means the Patriots will not sign Tinoisamoa, who has contract offers from other clubs.
Lenon, who has not missed a game during his seven NFL seasons, started all 16 contests for the Lions in each of the past three years. After playing primarily as a reserve and on special teams in Green Bay, he earned a spot in the Packers' starting lineup in 2005, starting 12 games in his final year with the franchise.
He should challenge for playing time in New England, and also figures to be a contributor on special teams units.
The former Richmond star totaled 100 tackles or more in each of the past two seasons, with a career-high 121 stops in 2008.
Lenon entered the NFL as an undrafted free agent with Green Bay in 2002. After four seasons with the Packers, he signed as a free agent with Detroit in 2006.
For his career, Lenon has 446 tackles, five sacks, two interceptions, 16 passes defensed, five forced fumbles and five recoveries. He is noted primarily as being a run-stuffer, and that should serve him well with his new team.
Lenon has appeared in 112 games, and started 64 of them. He is typical of New England's offseason signings in that he's a veteran player who will fill a very specific role with the club.
Len Pasquarelli is a senior NFL writer for ESPN.com.